This invention relates to the manufacture of carbon bodies for lining aluminum reduction cells, e.g. of the type wherein aluminum metal is produced from alumina by electrolysis of a fused electrolyte (frequently referred to as a bath) containing alumina and cryolite. More particularly, the invention is directed to methods of making lining bodies (including blocks and monolithic linings) incorporating carbon recovered from spent or used linings of such cells.
In a typical aluminum reduction cell (often called a pot), the carbon lining or potlining comprises a carbon structure which constitutes the interior side walls and bottom of the cell and is disposed as a very thick lining in a steel shell. This carbon lining functions as the cathode of the pot, usually by virtue of electrical connection to a multiplicity of steel cathode bars or the like embedded in the bottom portion, and as a result of current passed from one or more carbon anodes that extend downwardly into the bath, molten aluminum collects over the bottom and is withdrawn from time to time. The lining may be constructed of preformed carbon blocks, conventionally called prebake block, joined together with a carbon mix (e.g. particulate carbon with a suitable binder), or it can be made entirely of carbon mix suitably pressed into place, e.g. to form a completely monolithic structure, or it can be composed of various combinations of prebake blocks and monolithic carbon mass, for instance at different regions or layers.
Prebake blocks are conventionally formed by mixing a carbon aggregate with a conventional binder such as coal tar pitch or the like and hardening the mixture by baking in block form. Monolithic linings are made of similar mixtures also subjected to baking. Stated more generally, the lining (whether monolithic, or made wholly of partly of blocks) conventionally receives suitable heat treatment so that the carbon mix is baked in place and all the carbon becomes baked to a hard mass, before the cell is put in use.
During continuous operation of the reduction cell, the carbon lining is continually and progressively affected by the molten electrolyte and accompanying high temperature of the process, to the extent that the carbon lining absorbs a considerable quantity of material from the bath. Deterioration is an unavoidable consequence of the growth and disruption of the carbon structure, and final failure or near failure of the pot eventually occurs, being evidenced by such conditions as leakage of metal or molten bath, substantial distortion of the cathode structure, or iron pickup in the product metal. In particular, the carbon lining is penetrated by various materials and to varying degrees according to the age of the pot, the location of a given lining area in the sides or bottom and the specifically selected procedure and temperature of operation, as well as other factors.
The materials thus penetrating the lining may include molten metal, but chiefly comprise values (in combined form) of sodium and aluminum, usually including cryolite or other fluorides. The aluminum, except for its fluoride, carbide and nitride forms, is in considerable part present as aluminum oxide (alumina) and in some part as metal. Thus the bath ingredients, which are mainly cryolite (sodium aluminum fluoride) and alumina, with minor quantities of other salts such as fluorides of sodium and calcium as may be desired for special purposes, tend to be absorbed in their original or other combined states. Carbides and nitrides, notably of aluminum, are also formed during the aluminum reduction process, being so formed or deposited in the carbon lining. As is well known, a given pot will operate continuously for a rather long time, usually upwards of a year and often much longer, but eventually must be shut down, and have its carbon lining completely replaced.
Various procedures have heretofore been employed or proposed for the recovery of useful material from failed reduction cell linings. One such procedure, found particularly effective for recovery of useful carbon as well as fluoride and alumina values that can be reused in cells, is described in U.S. Pat. No. 3,635,408. In accordance with the process there disclosed, carbon lining removed after long, chemical-accumulating use in an aluminum reduction cell is crushed and hydrated with dry steam at a temperature insufficient to destroy the carbon, so that unwanted carbides and nitrides are eliminated and the lining material is conditioned for recovery of useful values, advantageously by classification thereafter into a coarse fraction providing carbon in reusable form and a fine fraction in which chemical material is reclaimed, such as alumina and fluorides suitable for use in the fused bath of a reduction cell.
As well be understood, carbon for potlining must be of relatively good quality and is therefore by no means inexpensive. Consequently, the process of the aforementioned patent is advantageous in recovering, from spent linings, carbon suitable for reuse in potlinings, i.e. as a substitute for conventional carbon lining material such as electrically calcined anthracite, kiln calcined anthracite, and metallurgical coke. Specifically, the recovered carbon may be used, either alone or in mixture with a proportion of conventional carbon lining materials such as those just mentioned, to constitute a carbonaceous aggregate (in suitable divided form) suitable for prebake blocks and monolithic linings. Desirable economy is achieved by the substitution of recovered carbon for some or all of the conventional new carbon material in such an aggregrate.
In accordance with conventional procedure, the aggregate comprising the recovered carbon (alone, or in mixture with new carbon material) is mixed with a conventional binder such as coal tar pitch, the binder constituting e.g. 16% of the mix, and baked to form hardened lining bodies. Although the recovered carbon contains residual contaminant chemicals, derived from the cell bath during its prior use in a reduction cell lining, the resultant bodies are found fully suitable for use as cell linings.
As stated, the dry-steam hydration procedure of the aforementioned patent is highly effective in providing a good recovery both of reusable carbon and of chemicals for the bath, but other hydration operations may alternatively be employed to treat spent lining material for recovery of carbon as well as re-usable chemicals therefrom, the recovered carbon material being in each case characterized by reduced content of undesired chemicals, in particular carbides and nitrides, as compared with the initial spent lining material.